Bottom Line:
This Fbn2(-/-) phenotype is accounted for by improper activation of latent TGF-β that selectively blunts expression of osterix, the transcriptional regulator of osteoblast maturation, and collagen I, the structural template for bone mineralization.Additional in vitro evidence excludes a direct role of microfibrils in supporting mineral deposition.Together, these findings identify the extracellular microfibrils as critical regulators of bone formation through the modulation of endogenous TGF-β and BMP signaling.

ABSTRACTExtracellular regulation of signaling by transforming growth factor (TGF)-β family members is emerging as a key aspect of organ formation and tissue remodeling. In this study, we demonstrate that fibrillin-1 and -2, the structural components of extracellular microfibrils, differentially regulate TGF-β and bone morphogenetic protein (BMP) bioavailability in bone. Fibrillin-2- (Fbn2(-/-)) mice display a low bone mass phenotype that is associated with reduced bone formation in vivo and impaired osteoblast maturation in vitro. This Fbn2(-/-) phenotype is accounted for by improper activation of latent TGF-β that selectively blunts expression of osterix, the transcriptional regulator of osteoblast maturation, and collagen I, the structural template for bone mineralization. Cultured osteoblasts from Fbn1(-/-) mice exhibit improper latent TGF-β activation as well, but mature faster because of increased availability of otherwise matrix-bound BMPs. Additional in vitro evidence excludes a direct role of microfibrils in supporting mineral deposition. Together, these findings identify the extracellular microfibrils as critical regulators of bone formation through the modulation of endogenous TGF-β and BMP signaling.

Mentions:
Although neonatal lethality of Fbn1−/− mice limits performing extensive analyses of bone formation (Carta et al., 2006), these mutant animals nonetheless enabled us to compare and contrast osteogenic differentiation on a matrix deficient for either fibrillin-1 or -2. Fbn1- cOb proliferated normally but, in contrast to Fbn2- cells, they yielded more mineral nodules than WT cultures (Fig. 6, A and B); they also displayed a modest increase in Osx expression, a significant up-regulation of Col1a2 and Bglap1, and normal Runx2 and Fbn2 activity (Fig. 6 C). In vivo levels of Col1a2 and Bglap1 (but not Osx) transcripts were appreciably higher than control, and collagen accumulation was slightly greater in mutant than WT bones (Fig. 6 D and Fig. S1 C). Moreover, AP-positive cells and mineral deposits appeared earlier and grew faster in Fbn1- than WT cOb cultures (Fig. S1 D). Collectively, these observations were consistent with the notion that loss of fibrillin-1 accelerates osteoblast maturation.

Mentions:
Although neonatal lethality of Fbn1−/− mice limits performing extensive analyses of bone formation (Carta et al., 2006), these mutant animals nonetheless enabled us to compare and contrast osteogenic differentiation on a matrix deficient for either fibrillin-1 or -2. Fbn1- cOb proliferated normally but, in contrast to Fbn2- cells, they yielded more mineral nodules than WT cultures (Fig. 6, A and B); they also displayed a modest increase in Osx expression, a significant up-regulation of Col1a2 and Bglap1, and normal Runx2 and Fbn2 activity (Fig. 6 C). In vivo levels of Col1a2 and Bglap1 (but not Osx) transcripts were appreciably higher than control, and collagen accumulation was slightly greater in mutant than WT bones (Fig. 6 D and Fig. S1 C). Moreover, AP-positive cells and mineral deposits appeared earlier and grew faster in Fbn1- than WT cOb cultures (Fig. S1 D). Collectively, these observations were consistent with the notion that loss of fibrillin-1 accelerates osteoblast maturation.

Bottom Line:
This Fbn2(-/-) phenotype is accounted for by improper activation of latent TGF-β that selectively blunts expression of osterix, the transcriptional regulator of osteoblast maturation, and collagen I, the structural template for bone mineralization.Additional in vitro evidence excludes a direct role of microfibrils in supporting mineral deposition.Together, these findings identify the extracellular microfibrils as critical regulators of bone formation through the modulation of endogenous TGF-β and BMP signaling.

ABSTRACTExtracellular regulation of signaling by transforming growth factor (TGF)-β family members is emerging as a key aspect of organ formation and tissue remodeling. In this study, we demonstrate that fibrillin-1 and -2, the structural components of extracellular microfibrils, differentially regulate TGF-β and bone morphogenetic protein (BMP) bioavailability in bone. Fibrillin-2- (Fbn2(-/-)) mice display a low bone mass phenotype that is associated with reduced bone formation in vivo and impaired osteoblast maturation in vitro. This Fbn2(-/-) phenotype is accounted for by improper activation of latent TGF-β that selectively blunts expression of osterix, the transcriptional regulator of osteoblast maturation, and collagen I, the structural template for bone mineralization. Cultured osteoblasts from Fbn1(-/-) mice exhibit improper latent TGF-β activation as well, but mature faster because of increased availability of otherwise matrix-bound BMPs. Additional in vitro evidence excludes a direct role of microfibrils in supporting mineral deposition. Together, these findings identify the extracellular microfibrils as critical regulators of bone formation through the modulation of endogenous TGF-β and BMP signaling.